Lithium-ion batteries have already seen widespread use as a high-capacity chemical power source in mobile communications, notebook computers, video cameras, photo cameras, and all other types of portable apparatus. They are also the prime target of many nations' research into power sources for electric vehicles and outer space applications, and have become a first choice in alternative energy sources.
Lithium iron phosphate (LiFePO4) is a hot research area as cathode active materials for lithium-ion batteries. Initial research indicated that LiFePO4 enjoyed the benefits of LiCoO2, LiNiO2, LiMnO4, and other materials: it does not contain precious metals, the original materials are inexpensive, and it can be created from widely available materials; it has a moderate voltage level (3.4V); it has good platform characteristics, stable voltage, and a high theoretical capacity (170 mAh/g); its structure is stable, and it is extremely safe (oxygen and phosphorous are held together with strong covalent bonds, making oxygen decomposition of materials difficult); it performs well under high temperatures and has good cycling ability; its volume shrinks during recharging, and it has good volume effect when paired with a carbon-based anode; it is compatible with most electrolyte solution systems, it is non-toxic, has good storage capacity, and can truly be considered as a green energy source.
However, LiFePO4 has poor conductive qualities, leading to a relatively low battery capacity and greatly limiting the usefulness of batteries constructed with such material for high current-density applications. According to the reports, current methods to improve or increase the conductivity of cathode active materials are mainly limited to surface mixing or coating the conducting carbon material or the conducting metallic particles in order to increase the inter-particulate electron conductivity of the original material. However, the electrical capacities of cathode active materials obtained through these methods are still rather low.
CN1797823A discloses a type of lithium iron phosphate cathode active material containing oxygen vacancies for use in rechargeable lithium-ion batteries. Its chemical formula is LiFe1-xMxPO4-yNz, where M represents Li, Na, K, Ag, or Cu; 0≦x≦0.1, 0≦y≦1, 0≦z≦0.5, x, y, and z are not all 0, and the following conditions are met: (1) when x=0 and z=0, 0≦y≦1; (2) when x=0 and z≠0, y≧3z/2; (3) when z=0 and x≠0, y≧x/2; (4) when x≠0 and z≠0, y≧x/2+3z/2. The goal of this patent application is to increase electronic and ionic conductivity, and thereby increase the capacity of rechargeable lithium-ion batteries constructed using this type of cathode active material.